[ticket/13315] Fix loss of CAPTCHA plugin config value.

The plugins were moved to phpbb/captcha/plugins/ after this migration was
created, thus the check is no longer valid and ends up resetting the value
prematurely.

PHPBB3-13315
[ticket/13315] Check for the includes/captcha/plugins/ directory instead.

PHPBB3-13315
Merge pull request #3148 from prototech/ticket/13315

[ticket/13315] Fix loss of CAPTCHA plugin config value.

* prototech/ticket/13315:
  [ticket/13315] Check for the includes/captcha/plugins/ directory instead.
  [ticket/13315] Fix loss of CAPTCHA plugin config value.